There is increasing demand for new technologies that decrease humanity's dependence on fossil fuels. With the challenges of finding and developing new reserves of gas and increasing constraints on carbon footprints, it is becoming more important to develop novel routes to obtain methane other than by extracting it from natural gas. An area of key interest is the use of biomass and solid refuse as a source of renewable carbon in the production of ‘biogas’ or ‘biomethane’. The use of such carbon sources may allow for the production and use of methane with more sustainable CO2 emissions when compared with natural gas.
Biomass-based feedstocks such as feedstocks containing lignocellulose (e.g. woody biomass, agricultural residues, forestry residues, residues from the wood products and pulp & paper industries) and municipal solid waste containing lignocellulosic material (e.g. old corrugated containers (OCC), waste paper, or food waste) are important feedstocks for non-fossil fuel derived sources of energy, due to their availability on a large scale. Lignocellulose comprises a mixture of lignin, cellulose and hemicelluloses in any proportion and usually also contains ash and moisture. Additionally, food waste contains starch, simple sugars, fats, oils and proteins.
Two main methods have been described in the art for the production of methane from biomass.
Firstly, biogas can be produced by anaerobic digestion of biodegradable materials with anaerobic microbes, which digest material inside a closed system. Anaerobic digestion methods typically require wet and easily digestible biomass as the feedstock to produce methane with high yields. The feedstock may be energy crops such as maize silage or biodegradable wastes including sewage sludge and food waste. The wet biomass used must be easy to digest to obtain high methane yields. That is, it must include relatively simple molecules such as sugars, starches, fats/oils and proteins. Conversion of hard, solid biomass such as woody biomass, agricultural residues, and lignocellulosic components of municipal solid waste, which are unfit for use as food or feed, to methane using the anaerobic digestion route is not practiced on a commercial scale. Literature reports, such as J-C Frigon and S. R. Guiot, Biofuels, Bioprod. Bioref. 2010, 4, 447-458, indicate that solid biomass feedstocks may be used but would require a pre-treatment before anaerobic digestion. The aim of such pre-treatment is to deconstruct the constituents of solid biomass and to convert cellulose/hemicellulose components into molecules that can be converted to methane by the microbial community by anaerobic processes. Some examples of suitable pretreatment are enzymatic hydrolysis, acid hydrolysis, solvent-based processes, steam explosion, ammonia recycle percolation, ammonia fiber explosion, lime treatment, and the OrganoSolv process. The lignocellulosic feedstock, after the pretreatment, is subjected to anaerobic microbial digestion and converted to a biogas. Biogas typically contains about 50-75 vol % methane, 25-50 vol % CO2, 1-5 vol % water vapor, 0-5 vol % nitrogen, small amounts of H2S(0-0.5 vol %) and NH3 (0-0.05 vol %), and trace amounts of H2 and CO. Lignin is typically not deconstructed well and not converted to biogas with high yields by the microbial action. It is necessary to separate methane from biogas and pressurize it before using the biomethane produced by anaerobic digestion as a substitute for natural gas.
Gasification, such as described in ‘Large scale production of biomethane from wood’ Van der Meijden, C. M.; Rabou, L. P. L. M; van der Drift, A.; Vreugdenhil, B. J.; Smit, R.; October 2011, Energy Research Centre of the Netherlands Report Number ECN-M-11-098, is another method available to produce methane from hard, solid biomass. To produce methane from biomass by gasification, medium temperature (about 800 to 900° C.) gasification is generally employed. Medium temperature gasification of biomass produces a ‘producer gas’, which contains mainly light hydrocarbons (CH4, C2H4) benzene, CO, CO2, H2, and water vapor, and contaminants such as dust (ash), tars, chlorides and sulfur compounds. Fluidized bed gasification processes produce gas which always contains some tar. Tar compounds are problematic for operation, as they condense into a high viscosity liquid as the gas is cooled. When the condensed tars combine with dust, the gas becomes very difficult to handle in the process equipment. Care must be taken to remove tars and dust from the process stream, and their removal often requires specialized process steps. Catalytic upgrading of tar- and dust-free producer gas into biomethane also requires deep removal of sulfur and chlorides, as these species are poisons for the methanation catalyst. Removal of sulfur and chlorine from producer gas is itself a multi-step process, with the final steps including the use of adsorbents such as ZnO for removal of sulfur down to ppb level. Finally, clean producer gas is sent to a pre-reformer, where larger hydrocarbons such as benzene are converted to a mixture of CO, CH4, CO2, H2O and H2. Conversion of the higher hydrocarbons into the lighter products makes the removal of carbon dioxide and the compression of the gas for methanation easier. After CO2 and H2O removal, the processed gas is converted into methane in the methanation section. The methanation is typically carried out in a fixed bed catalytic reactor using a nickel based catalyst. Gasification of biomass to produce synthetic natural gas is thus a multi-step process consisting of biomass gasification, tar separation, dust separation, separation of sulfur and chloride impurities, pre-reforming, CO2 separation followed by methanation and separation of the methane for injection into a methane grid.
An efficient method for processing biomass into high quality liquid fuels is described in WO2010117437, in the name of Gas Technology Institute. This method is directed to the production of liquid fuels in the diesel, gasoline and/or jet fuels range. While not being limited to any particular catalyst, exemplary catalysts for use in such a process include sulfided CoMo or NiMo catalysts based on metal oxide supports. The process may create some C1-C3 gases, and, in order to produce hydrogen to satisfy at least some of the hydrogen requirements of the process, these gases may be separated and sent to a reforming section.
There remains a need for effective catalysts with activity for producing methane in high yields from biomass-based feedstocks, particularly solid biomass, and especially such catalysts for use in processes that do not create a conflict with food production for humans and/or animals.